The present invention is directed to polarization-independent single-mode ridge waveguides.
Optical ridge waveguides are widely used as underlying waveguiding structures in integrated photonic devices and circuits based on semiconductor and other materials due to their ease of fabrication. One of the major shortcomings of the ridge waveguides is polarization sensitivity arising from the geometrical (or form) birefringence of the transverse configuration. A conventional shallow-etched ridge waveguide, such as depicted in FIG. 1 and generally designated as 100, typically includes a substrate 10, a lower cladding layer 20 deposited above the substrate 10, a guiding layer 30above the lower cladding layer 20, and an upper cladding layer 40 above the guiding layer 30. That waveguide 100 has relatively weak lateral confinement for the optical field, which results in severe leakage loss when a bend is introduced in the waveguide 100.
Deeply etched ridge waveguides, such as depicted in FIG. 2 and generally designated as 100xe2x80x2, provide strong lateral confinement and may be bent at small radius without significant leakage loss. An example of such known waveguides can be found in U.S. Pat. No. 5,926,496, which is hereby incorporated by reference in its entirety. Strong lateral confinement permits realization of photonic devices and circuits with much smaller dimensions so that high-performance and low-cost photonic integrated circuits with complex circuitry and a high level of integration may be constructed.
Strongly confined ridge waveguides are, however, highly polarization sensitive. The polarization sensitivity limits applications of the waveguides in fiber-optic communication systems, in which the polarization states of input optical signals to the photonic devices such as switches, add/drop multiplexers, and so on, may change randomly as a function of time and environmental conditions and are not predictable.
Several methodologies have been suggested to reduce and eliminate the polarization sensitivity for guided-wave optical devices in general. Such methods include polarization control, polarization scrambling, polarization diversity, and polarization independent waveguide design. All those methods, except for the polarization independent waveguide design, involve additional components and thus increase the complexity of the devices.
It is thus desirable to provide a ridge waveguide that overcomes the above-described shortcomings of the prior art.
The present invention is directed to a strongly confined ridge waveguide that provides substantially reduced polarization sensitivity, without significant compromise for other waveguide characteristics such as, for example, single-mode condition, and low propagation and bending losses for the fundamental mode. The present invention considers waveguide material composition and thickness for guiding and cladding layers, bend radius, ridge width and etch depth at which the modal indices of the fundamental TE and TM modes are equal. With those parameters, the losses (e.g., the imaginary parts of the modal indices) of the fundamental and first-order modes may be calculated. By considering the previously mentioned criteria, a low-loss, single-mode ridge waveguide may be constructed in accordance with the present invention having losses of the fundamental modes in the range of less than approximately 1.0 dB/mm, and losses of higher-order modes in the range of greater than approximately 10 dB/mm (thus providing a loss difference of at least approximately 10 dB/mm).
In a first embodiment of the present invention, a polarization-independent single-mode ridge waveguide comprises a lower cladding layer having a thickness, an upper cladding layer having a thickness, and a guiding layer having a thickness and being disposed between the lower and upper cladding layers. A ridge having a ridge width is defined longitudinally along the waveguide by the upper cladding layer and a part of the guiding layer. The ridge width ranges from approximately 1.0 mm to approximately 1.5 mm. An etching depth is defined by the ridge and ranges from approximately 1.7 mm to approximately 2.4 mm. A bend radius is defined along at least a part of a longitudinal length of the waveguide and ranges from approximately 0 mm to approximately 100 mm.
In a second embodiment of the present invention, a polarization-independent single-mode ridge waveguide for guiding an optical signal having a fundamental mode and at least a first higher-order mode comprises a lower cladding layer, an upper cladding layer having a thickness, and a guiding layer having a thickness and disposed between the lower and upper cladding layers. A ridge having a ridge width is defined longitudinally along the waveguide by the upper cladding layer and a part of the guiding layer. An etching depth is defined by said ridge, and a bend radius is defined along at least a part of a longitudinal length of the waveguide. The ridge width, etching depth, said bend radius each have a predetermined value to provide a difference in optical loss for the fundamental mode and for the at least first higher-order mode of at least approximately 10 dB/mm.
By proper choice of the etching depth, the total leakage loss of the higher-order modes may be maximized, while maintaining negligible loss for the fundamental mode. As will be discussed in more detail below, by proper choice of the ridge width and the etching depth for a given bending radius, it is possible to design a polarization-independent single-mode straight or bending waveguide with small leakage loss for the fundamental guided mode.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.